Correcting Nonuniform Illumination - Mathworks demo

Hi,

http://www.mathworks.in/products/image/examples.html?file=/products/demos/shipping/images/ipexrice.html

In the above link one of the method for correction of non uniform illumination has been explained. How to choose the following parameters

1. background = imopen(I,strel('disk',15));

Morphological opening operation - Radius - 15 ???

2. bw = bwareaopen(bw, 50);

Area opening - 50 ??? (connected component fewer than 50 pixels)